Understanding Mineral Fluorescence

Fluorescence is a fascinating phenomenon. The most common definition of fluorescence is "the property of emitting visible light during radiation by ultraviolet light". Most fluorescent minerals fluoresce under shortwave ultraviolet light—that is, light having a wavelength of 2537 angstroms.

This light will cause irritation and even graver consequences if shined for any length of time into the eyes. It can burn the retina of the eye, and for this reason a person should not look directly into a source of shortwave ultraviolet light.

Most of the fluorescent minerals are almost completely nondescript in ordinary lighting. They are drab and uninteresting. Yet, when placed in a darkened room, under the rays of shortwave or longwave ultraviolet light, they appear breathtakingly beautiful. The colors are vibrant, intense, and dramatic and very difficult to capture on color film.

The Science Behind the Glow

Different minerals fluoresce in different colors, and these colors are always constant for that particular mineral. In fact, fluorescence is a most accurate method of identifying certain minerals.

In order to understand fluorescence, it is first necessary to understand something about the physical structure of matter. Everything on earth is made up of small particles called molecules, which in turn are made up of atoms. The atoms consist of a nucleus and electrons. The nucleus remains in the center, and the electrons revolve around the nucleus, much as do the planets around our sun.

Radiation is a form of energy, and ultraviolet light is no exception. Many minerals are sensitive to the radiation of ultraviolet light, and these minerals fluoresce and phosphoresce under that energy. When ultraviolet light is shined upon a piece of one of these minerals, the light passes through some of the atoms. When some of the energy strikes an electron, the additional energy the electron absorbs from the light causes it to move to an orbit farther away from the nucleus, to a higher energy level within the atom. The movement of this electron into the larger orbit creates a void in the electron shell it vacated. This gap must be filled in order for the atom to maintain its electrical balance. The electrons closer to the nucleus do not have enough energy to travel out into a higher energy level, so an electron from a larger orbit must travel down into the vacant gap.

When the ultraviolet light is removed from the substance, all the electrons snap back to their normal positions in their normal orbits, and the visible light, or photons, ceases to be given off.

In some minerals, however, the electrons are slow to return to their normal orbits. Some take several hours, and some are known to take several years to return to normal. As long as the electrons are in the process of returning to their normal orbits, the mineral will continue to produce light. It is this afterglow, when the ultraviolet light is removed, that is called phosphorescence.

In order for a mineral to fluoresce, it must contain some impurities as an activator. Without these impurities the material will not fluoresce at all. The amount of impurity required to cause a mineral to become fluorescent is very small, and very critical—as critical, almost, as the amounts of energy required to hold the electrons in place in their orbits. The amount and the type of these impurities determine the intensity and the color of the fluorescence in that mineral.

Practical Applications of Fluorescence

Fluorescence has found many uses in industry and also in police work. One important use it has been put to is the tagging of postage stamps, to permit the rapid automatic sorting of mail. Airmail stamps are printed with an invisible ink which fluoresces one color, and the regular-mail stamps with an ink which fluoresces another color. The mail all mixed up, is run through a high-speed sorting machine which has photosensitive eyes to detect the color brought into the visible spectrum by the application of ultraviolet light. Those letters bearing stamps fluorescing in one color are kicked into one bin while the others are passed on to a second bin. Many thousands of man-hours each day are saved in the sorting of millions of pieces of mail.

Another most important application of fluorescence is in the location of various minerals, such as scheelite, a valuable tungsten ore. Tungsten is one of the metals vital to our economy, and deposits of it are detected with the aid of battery-operated ultraviolet lights used at night. Night prospecting with this apparatus has yielded much valuable material.

List of Fluorescent Minerals for Collectors

Some minerals fluoresce better under longwave ultraviolet light—3000 to 4000 angstroms in length—than under the shortwave ultraviolet light—2537 angstroms. Following is a list of minerals which fluoresce, together with a table stating whether they require short- or longwave light, and whether they also phosphoresce. In Franklin, New Jersey, is a mine from which a great number of very distinctive minerals are taken. It is probably the largest source in this country.

Not all of the minerals listed here are of interest to the collector, but many of them are.

There are certain abbreviations used in this list for words such as phosphorescent, fluorescent, etc., which are repeated often:

• ph = phosphorescent, phosphorescence, phosphoresce
• fl = fluorescent, fluorescence, fluoresce
• lw = longwave ultraviolet light, 3000-4000 angstroms
• sw = shortwave ultraviolet light, 2537 angstroms

• adamite: Green under sw, paler under lw
• amber: Fl blue-white under lw
• anglesite: Yellow under sw
• aragonite: Green fl produced by sw
• axinite: Fl deep red under sw and faintly under lw. A long-lived ph is also present.
• barite: White or cream color under lw
• barylite: blue-white fl under sw.
• benitoite: Fl brightblue under sw.
• calcite: fl almost every color of the rainbow. From Texas comes a species that fl and ph blue under sw and pink under lw
• calcium larsenite: A rare mineral found only at Franklin, New Jersey. Fl brilliant chartreuse under sw and dull yellow under lw
• calomel: Fl brick red under sw.
• celestite: Blue under both sw and lw; has a greenish-white ph.
• cerussite: Yellow fl under lw.
• chondrodite: A bright golden yellow, and yellow-orange to buff under sw.
• clinohedrite: Fl a golden brown under sw, with ph also present.
• colemanite: White or cream under both sw ad lw, and also some ph.
• corundum: Deep red under lw.
• curtisite: Sw produces white, cream, and pale green swirls and patterns in this mineral
• deweylite: White under both lw and sw
• diamond: Fl green, orange, red, and blue under lw
• dumortierite: Blue-white fl under sw
• eucryptite: Cerise under sw. A most unusual and beautiful color
• fluorite: Fl brilliant blue under lw. Paler under sw
• hackmanite: Brilliant apricot under lw, pale apricot under sw
• hyalite: Brilliant green under sw
• hydrozincite: Blue-white under sw
• manganapatite: Buff-brown to bright golden yellow under sw
• margarosanite: A brilliant blue-white fl under sw
• norbergite: Fl bright-yellow to dull buff under sw
• opal: green under sw, lesser intensity under lw
• pectolite: Bright-yellow to cream under sw, gold ph under sw
• petalite: Fl white under lw and sw
• phlogopite: Buff-yellow under sw
• phosgenite: Yellow under sw
• powellite: Cream or golden under sw
• scapolite (wernerite): Brilliant yellow under lw. This is one of the most spectacular fl minerals activated by lw. Also ph weakly
• sheelite: Bright blue under sw
• sodalite: Golden-brown under lw
• sphalerite: Orange or golden-brown under lw
• spinel: Brilliant red under lw
• stolzite: Fl greenish-white under both lw and sw
• svabite: Fl a bright golden-brown under sw
• talc: Fl creamy white or pale greenish-white under sw
• terlinguaite: Bright yellow under sw
• tremolite: Orange under both sw and lw
• willemite: Very brilliant green under sw
• witherite: very brilliant green under sw
• wollastonite: Brilliant orange fl under sw. This rare mineral is found at Franklin, New Jersey.
• zircon: Golden-yellow to brown under sw.

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